1. Field of the Invention
The present invention relates to a capacitor circuit in which multiple capacitor lines having capacitors connected thereto are connected in parallel with each other. In particular, the present invention relates to a smoothing capacitor circuit in a power conversion circuit that converts alternating-current power into direct-current power.
2. Description of the Related Art
Inverter circuits are used as circuits that provide alternating-current power to a motor in an electric car, hybrid car, or other suitable devices. Inverter circuits generally include a battery, which is a direct-current power supply, a conversion circuit, which converts direct-current power into alternating-current power, and a smoothing capacitor circuit, which is connected between the battery and the conversion circuit (see, for example, Japanese Unexamined Patent Application Publication No. 11-98852). Film capacitors primarily made of a polypropylene film (P.P.) are currently being used as capacitors for use in a smoothing capacitor circuit.
In film capacitors, the allowable ripple current per unit volume or per unit capacitance is limited by the operating temperature. Thus, in order for an inverter circuit to meet the requirements of the allowable ripple current in a high-current system, such as a car-mounted system, under an operating environment (operating temperature, etc.) that is harsh for electronic components, its smoothing circuit must ensure a capacitance equal to or greater than a required capacitance. For this reason, a capacitor circuit in which a film capacitor has an increased size or a capacitor circuit in which multiple film capacitors are connected in parallel with each other is used. Accordingly, the module of a conventional capacitor circuit that includes only a film capacitor has an increased size.
A capacitor circuit in which a ceramic capacitor having a high allowable ripple current, a high heat resistance, a low ESR, and a low ESL is combined with a film capacitor has been proposed as a capacitor circuit for improving the allowable ripple current without excessively increasing the size of the enclosure.
FIG. 11 is an equivalent circuit diagram of a smoothing capacitor circuit in which a plurality of capacitors are connected in parallel with each other.
As shown in FIG. 11, a smoothing capacitor circuit 101 includes a film capacitor 111 and a ceramic capacitor 112 that are connected in parallel with each other. The film capacitor 111 has a capacitance C111, as well as an equivalent series resistance (ESR) R111 and an equivalent series inductance (ESL) L111. A feature of the ceramic capacitor 112 is to have a capacitance C112, as well as characteristics of an equivalent series resistance (ESR) R112 and an equivalent series inductance (ESL) L112.
Unfortunately, in the above-described capacitor circuit in which the film capacitor and the ceramic capacitor are connected in parallel with each other, the film capacitor has impedance characteristics that are different from those of the ceramic capacitor.
FIG. 12A is a graph showing the respective impedance-frequency characteristics of the film capacitor 111 circuit and the ceramic capacitor 112 circuit shown in FIG. 11 and the combined impedance-frequency characteristic of the capacitor circuit 101. FIG. 12B is a graph showing the respective current-frequency characteristics of the film capacitor 111 circuit and the ceramic capacitor 112 circuit exhibited when an external current having an RMS value of 1 A was applied to the capacitor circuit 101.
FIG. 13A is a graph showing the frequency spectrum of the current of the film capacitor 111 circuit. FIG. 13B is a graph showing the frequency spectrum of the current of the ceramic capacitor 112 circuit.
The simulations whose results are shown in FIGS. 12A to 13B were conducted under the following conditions. The capacitance C111 of the film capacitor 111 is about 1160 μF, the series resistance component R111 is about 0.75 mΩ, and the series inductance component L111 is about 20 nH. The capacitance C112 of the ceramic capacitor 112 is about 40 μF, the series resistance component R112 is about 2 mΩ, and the series inductance component L112 is about 2 nH. An alternating current having an rms value of about 1 A and about 1 kHz to about 10 MHz from a constant current source was applied to the capacitor circuit 101 having these element parameters.
As shown in FIG. 12A, the film capacitor 111 has an impedance-frequency characteristic different from that of the ceramic capacitor 112. The parallel connection between the capacitors having the different impedance-frequency characteristics, as described above, causes parallel resonance, since the magnitude of inductive reactance and that of capacitive reactance are matched in the closed circuit provided by the two lines. For example, in FIG. 12A, parallel resonance occurs around 200 kHz. The difference between this parallel resonant frequency and the series resonant frequencies of the lines generates a resonant current, which circulates through the closed circuit composed of the circuit in which the two capacitors are connected in parallel with each other.
If the ripple current flowing from the switching circuit includes the parallel resonant frequency components, the parallel resonant frequency components of the ripple current are amplified so that each capacitor current becomes a significant overcurrent as shown in the current value of FIG. 12B or in the spectrum of about 100 kHz to about 200 kHz of FIGS. 13A and 13B. The overcurrent of the parallel resonant frequency components heats the film capacitor, which reduces the allowable ripple current of the smoothing capacitor circuit.